index.js

let app = require('express')();
let http = require('http').Server(app);
let io = require('socket.io')(http);
 
io.on('connection', (socket) => {
  
    socket.on('disconnect', function(){
        io.emit('users-changed', {user: socket.nickname, event: 'left'});   
    });

    socket.on('set-nickname', (nickname) => {
        socket.nickname = nickname;
        io.emit('users-changed', {user: nickname, event: 'joined'});    
    });

    socket.on('add-message', (message) => {
        io.emit('message', {text: message.text, from: socket.nickname, created: new Date()});    
    });
});
 
var port = process.env.PORT || 3001;
 
http.listen(port, function(){
   console.log('listening in http://localhost:' + port);
});